Mybatis插件原理分析（一）

柔光的暖阳◎ 2022-02-23 11:28 370阅读 0赞

我们首先介绍一下Mybatis插件相关的几个类，并对源码进行了简单的分析。

Mybatis插件相关的接口或类有：Intercept、InterceptChain、Plugin和Invocation，这几个接口或类实现了整个Mybatis插件流程。

Interceptor：一个接口，是实现自己功能需要实现的接口

源码如下：

/**
     * @author Clinton Begin
     */
    public interface Interceptor {
     
      //在此方法中实现自己需要的功能，最后执行invocation.proceed()方法，实际就是调用method.invoke(target, args)方法，调用代理类
      Object intercept(Invocation invocation) throws Throwable;
     
      //这个方法是将target生成代理类
      Object plugin(Object target);
      //在xml中注册Intercept是配置一些属性
      void setProperties(Properties properties);
     
    }

InterceptorChain：有一个List<Interceptor> interceptors变量，来保存所有Interceptor的实现类

源码如下：

/**
     * @author Clinton Begin
     */
    public class InterceptorChain {
     
      //插件拦截器链
      private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
     
      //把target变成代理类，这样在运行target方法之前需要运行Plugin的invoke方法
      public Object pluginAll(Object target) {
        for (Interceptor interceptor : interceptors) {
          target = interceptor.plugin(target);
        }
        return target;
      }
     
      public void addInterceptor(Interceptor interceptor) {
        interceptors.add(interceptor);
      }
      
      public List<Interceptor> getInterceptors() {
        return Collections.unmodifiableList(interceptors);
      }
     
    }

Invocation：一个比较简单的类，主要的功能就是根据构造函数类执行代理类

源码如下：

/**
     * @author Clinton Begin
     */
    public class Invocation {
     
      private Object target;
      private Method method;
      private Object[] args;
     
      public Invocation(Object target, Method method, Object[] args) {
        this.target = target;
        this.method = method;
        this.args = args;
      }
     
      public Object getTarget() {
        return target;
      }
     
      public Method getMethod() {
        return method;
      }
     
      public Object[] getArgs() {
        return args;
      }
      //其实mybatis的Interceptor最终还是调用的method.invoke方法
      public Object proceed() throws InvocationTargetException, IllegalAccessException {
        return method.invoke(target, args);
      }
     
    }

Plugin：Mybatis插件的核心类，它实现了代理接口InvocationHandler，是一个代理类

源码详解如下：

/**
     * @author Clinton Begin
     */
    //这个类是Mybatis拦截器的核心,大家可以看到该类继承了InvocationHandler
    //又是JDK动态代理机制
    public class Plugin implements InvocationHandler {
     
      //目标对象
      private Object target;
      //拦截器
      private Interceptor interceptor;
      //记录需要被拦截的类与方法
      private Map<Class<?>, Set<Method>> signatureMap;
     
      private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
        this.target = target;
        this.interceptor = interceptor;
        this.signatureMap = signatureMap;
      }
     
      //一个静态方法,对一个目标对象进行包装，生成代理类。
      public static Object wrap(Object target, Interceptor interceptor) {
    	//首先根据interceptor上面定义的注解 获取需要拦截的信息
        Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
        Class<?> type = target.getClass();
        Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    	//如果长度为>0 则返回代理类 否则不做处理
        if (interfaces.length > 0) {
    	  //创建JDK动态代理对象
          return Proxy.newProxyInstance(
              type.getClassLoader(),
              interfaces,
              new Plugin(target, interceptor, signatureMap));
        }
        return target;
      }
     
      //在执行Executor、ParameterHandler、ResultSetHandler和StatementHandler的实现类的方法时会调用这个方法
      @Override
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
    	  //通过method参数定义的类 去signatureMap当中查询需要拦截的方法集合
          Set<Method> methods = signatureMap.get(method.getDeclaringClass());
    	  //判断是否是需要拦截的方法，如果需要拦截的话就执行实现的Interceptor的intercept方法，执行完之后还是会执行method.invoke方法，不过是放到interceptor实现类中去实现了
          if (methods != null && methods.contains(method)) {
            return interceptor.intercept(new Invocation(target, method, args));
          }
    	  //不拦截 直接通过目标对象调用方法
          return method.invoke(target, args);
        } catch (Exception e) {
          throw ExceptionUtil.unwrapThrowable(e);
        }
      }
      //根据拦截器接口（Interceptor）实现类上面的注解获取相关信息
      private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
    	//获取注解信息
        Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
        // issue #251
    	//为空则抛出异常
        if (interceptsAnnotation == null) {
          throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());      
        }
    	//获得Signature注解信息
        Signature[] sigs = interceptsAnnotation.value();
        Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
    	//循环注解信息
        for (Signature sig : sigs) {
    	  //根据Signature注解定义的type信息去signatureMap当中查询需要拦截方法的集合
          Set<Method> methods = signatureMap.get(sig.type());
    	  //第一次肯定为null 就创建一个并放入signatureMap
          if (methods == null) {
            methods = new HashSet<Method>();
            signatureMap.put(sig.type(), methods);
          }
          try {
    		//找到sig.type当中定义的方法 并加入到集合
            Method method = sig.type().getMethod(sig.method(), sig.args());
            methods.add(method);
          } catch (NoSuchMethodException e) {
            throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
          }
        }
        return signatureMap;
      }
      //根据对象类型与signatureMap获取接口信息
      private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
        Set<Class<?>> interfaces = new HashSet<Class<?>>();
    	//循环type类型的接口信息 如果该类型存在与signatureMap当中则加入到set当中去
        while (type != null) {
          for (Class<?> c : type.getInterfaces()) {
            if (signatureMap.containsKey(c)) {
              interfaces.add(c);
            }
          }
          type = type.getSuperclass();
        }
    	//转换为数组返回
        return interfaces.toArray(new Class<?>[interfaces.size()]);
      }
     
    }

对Mybatis插件有一定了解的人应该知道，插件拦截的类是Executor、ParameterHandler、ResultSetHandler和StatementHandler的实现类，为什么会这样呢？在Configuration类中看一下代码就明白了，可以看到在初始化Executor、ParameterHandler、ResultSetHandler和StatementHandler都会调用interceptorChain.pluginAll（）这个函数，其实这样之后各个接口的实现类就被代理类生成为目标类了。

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
        ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
        parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
        return parameterHandler;
      }
     
      public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
          ResultHandler resultHandler, BoundSql boundSql) {
        ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
        resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
        return resultSetHandler;
      }
     
      public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
        StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
        statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
        return statementHandler;
      }
     
      public Executor newExecutor(Transaction transaction) {
        return newExecutor(transaction, defaultExecutorType);
      }
     
      public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
        executorType = executorType == null ? defaultExecutorType : executorType;
        executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
        Executor executor;
        if (ExecutorType.BATCH == executorType) {
          executor = new BatchExecutor(this, transaction);
        } else if (ExecutorType.REUSE == executorType) {
          executor = new ReuseExecutor(this, transaction);
        } else {
          executor = new SimpleExecutor(this, transaction);
        }
        if (cacheEnabled) {
          executor = new CachingExecutor(executor);
        }
        executor = (Executor) interceptorChain.pluginAll(executor);
        return executor;
      }

本文原文地址：[https://blog.csdn.net/qq924862077/article/details/52588332][https_blog.csdn.net_qq924862077_article_details_52588332]

[https_blog.csdn.net_qq924862077_article_details_52588332]: https://blog.csdn.net/qq924862077/article/details/52588332